Anti-Reflective Coating with High Refractive Index Material at Air Interface

ABSTRACT

An optical element comprising a transparent substrate and an anti-reflective coating, wherein the anti-reflective coating further comprises at least a transparent, high refractive index layer and a transparent, low refractive index layer, wherein the high refractive index layer is in contact with the low refractive index layer; and wherein the high refractive index layer is situated at an interface between the anti-reflective coating and air. Further, the low refractive index layer may be silicon oxide; the high refractive index layer may be tantalum oxide or silicon nitride.

FIELD OF INVENTION

The current invention relates to anti-reflective (AR) coating of an image sensor. In particular, the anti-reflective coating is a multi-layer coating, which alternates between a harder, high refractive index material and softer, low refractive index material, with the high refractive index material situated at the air interface between the AR coating and air.

BACKGROUND

AR coating is widely used in image sensors. It is generally applied on top of a cover glass or a lens, and is situated with its air interface facing incoming light. As shown in FIG. 9, a packaged complementary metal oxide semiconductor (CMOS) image sensor package 900 includes a cover glass 910, a CMOS image sensor (CIS) device 930 that comprises microlens (not shown), substrate, photo detection regions (not shown), and metal connect regions (not shown), dams 940, and solder balls 950. The top of the cover glass 910 is coated with an AR coating 920, which is situated with its air interface facing incoming light.

One type of AR coating is a multi-layer coating, consisting of multiple layers of high and low refractive index materials. Based on the principle of destructive interference, the multi-layer AR coating works to reduce light reflection to be within only a few percent of incoming light, so that optical transmission is enhanced.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is the cross sectional view of a multi-layer anti-reflective (AR) coating, showing the conventional design of placing a low-index material at the air interface.

FIG. 2 is the cross sectional view of a multi-layer AR coating, with a high-index material at the air interface.

FIG. 3 is an actual scanning electron microscope (SEM) image of a cross section of an AR coating, showing alternating layers of tantalum oxide Ta₂O₅ and silicon dioxide SiO₂, with the high-index Ta₂O₅ layer situated at the air interface.

FIG. 4A is a wavelength vs. transmittance measurement plot of a multi-layer Ta₂O₅/SiO₂ AR coating, with a one nanometer high-index Ta₂O₅ outermost layer situated at the air interface.

FIG. 4B is a wavelength vs. reflectance measurement plot of a multi-layer Ta₂O₅/SiO₂ AR coating, with a one nanometer high-index Ta₂O₅ outermost layer situated at the air interface.

FIG. 5A is a plot of wavelength vs. transmittance of a multi-layer Ta₂O₅/SiO₂ AR coating, with the high-index Ta₂O₅ outermost layer situated at the air interface. Transmission performances of two thicknesses of the outermost Ta₂O₅ layer are compared.

FIG. 5B is a plot of wavelength vs. reflectance of a multi-layer Ta₂O₅/SiO₂ AR coating, with the high-index Ta₂O₅ outermost layer situated at the air interface. Reflectance performances of two thicknesses of the outermost Ta₂O₅ layer are compared.

FIG. 6A is a plot of wavelength vs. transmittance of a multi-layer AR coating, with the high-index outermost layer situated at the air interface. Transmission performances of two high-index materials, Ta₂O₅ and Si₃N₄, at the outermost layer are compared.

FIG. 6B is a plot of wavelength vs. reflectance of a multi-layer AR coating, with the high-index outermost layer situated at the air interface. Reflectance performances of two high-index materials, Ta₂O₅ and Si₃N₄, at the outermost layer are compared.

FIG. 7 is a schematic drawing of an image sensor package including an AR coated cover glass, showing the AR coatings on both sides of the cover glass.

FIG. 8 shows a process of forming AR coating on a bare glass substrate, including the main steps of the process.

FIG. 9 shows an image sensor package, with an AR coated cover glass.

DETAILED DESCRIPTION

Embodiments disclosed below describe an AR coated cover glass as part of an image sensor package. Particular embodiments relate to multi-layer AR coatings, with alternating layers of high and low refractive index materials. In some embodiments, a high index material layer is the outermost layer, situated at the air interface.

An image sensor's optical elements, such as its cover glass and lens, are generally made of transparent, glass (or plastic) materials. These materials generally have a reflectance of several percent (e.g., 4%), i.e., they reflect a certain percentage (e.g., 4%) of the incoming light. To reduce the reflectance and increase the transmittance (which is defined as 100% minus the reflectance), a relatively low refractive index material may be used to coat the optical elements. This is similar to the practice of index matching.

The general principle of light reflection is described below.

Generally speaking, a material's refractive index increases with its density. Air has a density of 0.0013 g/cm3, and a refractive index of 1; silicon oxide (SiO2) has a higher density of 2.65 g/cm3, and a higher index of 1.46. tantalum oxide (Ta₂O₅) has an even higher density of 8.2 g/cm3, and an even higher index of 2.13.

At the air interface of a dense material (such as glass), one the side of air, the refractive index of air is n₀=1; on the dense material side, the refractive index of the dense material is always n₁>1. The reflectance R of the incoming light (incoming air is defined as coming inside the air medium and reflecting off the dense material surface) may be calculated with the following formula:

$R = {\left( \frac{n_{0} - n_{1}}{n_{0} + n_{1}} \right)^{2} \times 100\%}$

For example, if the dense material is SiO₂, with its refractive index of 1.46, the reflectance will be 3.5%, which is lower than the 4% glass reflectance. If the material is an even denser material such as Ta₂O₅, with its high refractive index of 2.13, the reflectance will be 13%, which is much higher than the 4% glass reflectance.

One type of AR coating may be a single layer of low-index material (such as SiO₂). Another type of AR coating may have multiple layers of transparent materials. For a multi-layer AR coating, multiple alternating layers of two transparent materials, e.g., SiO₂ and Ta₂O₅, form a composite, layered AR coating. Both materials have refractive indexes that are greater than that of the air, and the refractive index of SiO₂ is less than that of Ta₂O₅. Conventionally, SiO₂ is referred to as a low-index material, and Ta₂O₅ is referred to as a high-index material.

FIG. 1 is the cross sectional view of an optical element 100 (such as a cover glass), including a bare glass substrate 120, which is coated with a multi-layer AR coating 110. The multi-layer AR coating 110 includes alternating layers, numbered as 101, 102, etc., wherein the innermost layer 101 is situated directly above bare glass substrate 120. The numbering of a layer goes up as the layer is further away from the bare glass substrate 120. For a 19 layer AR coating, the highest numbered layer 119 is the outermost layer, which is situated at the air interface.

The layers 101, 102, . . . , and 119 are alternating layers of the low-index SiO₂ and the high-index Ta₂O₅. As incoming light first passes through the air interface, and then passes through each of the multiple layers, reflection occurs at each interface, i.e., first at the interface between air and layer 119, then at the interface between layers 119 and 118, . . . , at the interface between layers 102 and 101, and finally at the interface between layer 101 and the bare glass substrate 120. Appropriate design of both the thickness and the refractive index of each layer is made to promote destructive interference to occur for each reflection, thereby rendering the overall reflection of the AR coating to be a low value.

Theoretically speaking, for the aforementioned multi-layer AR coating, the reflectance at a SiO₂—Ta₂O₅ interface is about 3.5%. At the interface between air and the outermost layer 119, the reflectance depends on the refractive index of that outermost layer 119. FIG. 1 shows a conventional design that places the low-index SiO₂ as the outermost layer 119 at the air interface. This is the conventional design, because the air-SiO₂ interface has a reflectance of only 3.5%. This low reflectance will help to keep the overall reflectance of the multi-layer AR coating at a low value. In contrast, an air-Ta₂O₅ interface has a 13% reflectance. If the outermost layer 119 is the high-index Ta₂O₅, then it will be difficult to keep the overall multi-layer AR coating reflectance to a low value, due to the high reflectance at the air-Ta₂O₅ interface.

Despite the conventional wisdom, positioning a SiO₂ layer as the outermost layer at the air interface has its shortcoming, in part because SiO₂ has a low hardness level (Mohs hardness index for SiO₂ is around 6 to 7). This renders the AR coating susceptible to scratch damage. Even when the top SiO₂ layer thickness is increased, the scratch problem still persists. For example, a pencil scratch test uses different hardness pencils (1H to 9H, wherein a higher H number represents a harder pencil lead) to scratch a multi-layer AR coated cover glass. When the outermost AR layer is SiO₂ (the design in FIG. 1), the pencil scratch test fails at 2H and higher. A solution to this scratch damage problem is to place the harder Ta₂O₅ (Mohs hardness index is around 9.2) at the outermost air-interface layer, but additional design features need to be implemented, as disclosed below.

FIG. 2 is the cross sectional view of another type of multi-layer AR coating, which has a high-index material (e.g., Ta₂O₅) positioned at the air interface. More specifically, an optical element 200 (such as cover glass) includes a bare glass substrate 220, and a multi-layer AR coating 210 that coats the bare glass substrate 220. The multi-layer AR coating 210 includes alternating layers, numbered as 201, 202, etc., with the innermost layer 201 situated directly above bare glass substrate 220 as shown. The numbering of a layer goes up as the layer is further away from the bare glass substrate 220. For a 19 layer AR coating, the highest numbered layer 219 is the outermost layer, and is situated at the air interface.

In this embodiment, the innermost (first) layer 201 is Ta₂O₅, the next (second) layer 202 is SiO₂, and so on. The layers alternate between SiO₂ and Ta₂O₅. For the outer layers near the air interface, the eighteenth layer 218 is SiO₂, whereas the outermost nineteenth layer 219 is Ta₂O₅. Since Ta₂O₅ (9.2 Mohs hardness index) is much harder than SiO₂ (6-7 Mohs hardness index), optical element design 200 in FIG. 2 (whose outermost layer is Ta₂O₅) is more scratch resistant than optical element design 100 in FIG. 1 (whose outermost layer is SiO₂). Notably, Ta₂O₅ has a higher refractive index (n=2.13) than SiO₂ (n=1.46), making the outermost layer 219 relatively more susceptible for reflection. This issue is dealt with, in part, by controlling the outermost layer thickness to be around 1 to 2 nm, along with additional design features to control the other layer thicknesses to be around certain values, as disclosed below.

Table 1 shows an AR coating design wherein the outermost nineteenth layer is a one nanometer Ta₂O₅ layer. It shows the inner most Layer 1 (layer 201 in FIG. 2) as an 11.9 nm Ta₂O₅ layer, the next Layer 2 (layer 202 in FIG. 2) as a 49.6 nm SiO₂ layer, and so on. Near the air interface, the eighteenth Layer 18 (layer 218 in FIG. 2) is an 85.3 nm SiO₂ layer, and the outermost nineteenth Layer 19 (layer 219 in FIG. 2) is a one nanometer (with a 5-10% variation) Ta₂O₅ layer.

TABLE 1 Multi-layer AR Coating Design Layer No. Material Thickness (nm) Layer 1 Ta₂O₅ 11.9 Layer 2 SiO₂ 49.6 Layer 3 Ta₂O₅ 25.8 Layer 4 SiO₂ 5.2 Layer 5 Ta₂O₅ 7.9 Layer 6 SiO₂ 11 Layer 7 Ta₂O₅ 77.4 Layer 8 SiO₂ 11 Layer 9 Ta₂O₅ 31.7 Layer 10 SiO₂ 4.3 Layer 11 Ta₂O₅ 5.9 Layer 12 SiO₂ 81.3 Layer 13 Ta₂O₅ 7.94 Layer 14 SiO₂ 61.5 Layer 15 Ta₂O₅ 43.7 Layer 16 SiO₂ 5.1 Layer 17 Ta₂O₅ 65.5 Layer 18 SiO₂ 85.3 Layer 19 Ta₂O₅ 1

FIG. 3 is a perspective view of an actual scanning electron microscope (SEM) image showing a cross section of the AR coating as described in Table 1. A cover glass, coated with a multi-layer AR coating as described in Table 1, is covered with an encapsulating agent (such as epoxy, for SEM imaging purpose only), and then cross sectioned for SEM. FIG. 3 shows alternating layers of tantalum oxide Ta2O5 (lighter colored layers) and silicon dioxide SiO2 (darker colored layers). Specifically, on top of bare glass substrate 320 is the first layer 301 of Ta2O5, with a thickness of 11.9 nm. Next is the second layer 302 of SiO2, with a thickness of 49.6 nm. A third layer 303 of Ta2O5, with a thickness of 25.8 nm, is on top of the second layer 302, and a fourth layer 304 of SiO2, with a thickness of 5.2 nm, is on top of the third layer 303. Alternating layers of Ta2O5 and SiO2 as described in Table 1 are shown in FIG. 3. Near the top (air interface) is the eighteenth layer 318 of SiO2, with a thickness of 85.3 nm. An outermost, nineteenth layer 319 is Ta2O5, with a thickness of only one nanometer. An encapsulating layer of epoxy 330 lies on top of the top Ta2O5 layer 319, and partially obscures this very thin, nineteenth layer 319 of Ta2O5.

This design, wherein the outermost layer is a thin (1-2 nm) Ta2O5 layer, has several advantages.

First, it works well to protect the AR coating from scratch damage. When the same pencil scratch test as described above is applied to this AR coating (e.g., the design in Table 1 and FIG. 3), the AR coating is able to pass even the hardest 9H pencil test without being scratched.

Second, it allows for high transmittance and low reflection, which are attributes of a good AR coating performance. FIG. 4A is a wavelength vs. transmittance measurement plot of this AR film design. It shows a high transmittance (about 99%) for a wavelength range of about 380-700 nm (almost the entire visible spectrum). FIG. 4B is a wavelength vs. reflectance measurement plot of this AR film design. It shows a low reflectance (below 0.5%) for a wavelength range of about 380-700 nm.

Third, this design provides for high transmittance and low reflectance even for low wavelength values of below 400 nm, which is close to the ultraviolet domain.

Notably, the outermost layer of Ta₂O₅ needs to very thin, generally about 1-2 nm (with a 5-10% variation). If this layer thickness is below 1 nm, it will be relatively difficult to precisely form such a thin layer with currently market-available coating technology such as vapor deposition and sputtering. Also, the protection that this thin Ta₂O₅ may offer against scratch damage will be reduced. On the other hand, if this Ta₂O₅ layer thickness is increased (e.g., 5 nm or more), performance of the entire multi-layer AR coating will diminish significantly, because transmittance will decrease, and reflectance will increase to approach unacceptable levels. In order for an AR coating to perform well, transmittance should be kept high and reflectance should be kept low. The following data shows that when the outermost layer of Ta₂O₅ is above 2 nm, AR coating performance will be substantially diminished.

FIG. 5A is a wavelength vs. transmittance measurement plot that compares a performance curve 501 of an AR coating with a 1 nm Ta₂O₅ outermost layer with a performance curve 502 of an AR coating with a 2 nm Ta₂O₅ outermost layer. Curve 501 shows that when the outermost Ta₂O₅ layer is 1 nm, the AR film's transmittance is above 99% for the 380-700 nm visible wavelength range. In contrast, curve 502 shows that when the outermost Ta₂O₅ layer is increased to 2 nm, the AR film's transmittance is substantially reduced to about 97%. FIG. 5B is a wavelength vs. reflectance measurement plot that compares a performance curve 503 of an AR coating with a 1 nm Ta₂O₅ outermost layer with a performance curve 504 of an AR coating with a 2 nm Ta₂O₅ top layer. Curve 503 shows that when the topmost Ta₂O₅ layer is 1 nm, the AR film's reflectance is below 0.5% for the 380-700 nm visible wavelength range. In contrast, curve 504 shows that when the outermost Ta₂O₅ layer is increased to 2 nm, the AR film's reflectance is substantially increased to over 2%. These data support keeping the outermost Ta₂O₅ layer thickness to be ideally less than or equal to 2 nm.

Besides Ta₂O₅, silicon nitride (Si₃N₄) may also be used as an outermost layer. Si₃N₄ has a hardness (Mohs index=8.5) that is significantly higher than that of SiO₂, and is similar to that of Ta₂O₅. In addition, the refractive index of Si₃N₄ is around 2.016, which is similar to Ta₂O₅. Using Si₃N₄ to replace Ta₂O₅ will form a multi-layer AR coating that has a similar optical performance as the previously disclosed multi-layer Ta₂O₅/SiO₂ AR coating.

FIG. 6A is a wavelength vs. transmittance measurement plot that shows a performance curve 601 of an AR coating with a 1 nm Ta₂O₅ outermost layer being similar to a performance curve 602 of an AR coating with a 1 nm Si₃N₄ outermost layer. FIG. 6B is a wavelength vs. reflectance measurement plot that shows a performance curve 603 of an AR coating with a 1 nm Ta₂O₅ outermost layer being similar to a performance curve 602 of an AR coating with a 1 nm Si₃N₄ outermost layer. These data support using Si₃N₄ in place of Ta₂O₅ to form a multi-layer AR coating with SiO₂. It should be noted that Si₃N₄ is less resistant to chemical erosion than Ta₂O₅.

As disclosed above, a special, multi-layer (e.g., alternating Ta₂O₅/SiO₂ layers) AR coating with a thin (e.g., 1-2 nm, with a 5-10% variation), high refractive index material (e.g., Ta₂O₅) at the outermost layer (i.e., at the air interface) may be used to achieve good anti-reflection performance. This special AR coating may be applied to the outer surface of a cover glass of an image sensor package, as shown in FIG. 9. The AR coating may also be applied to both sides of a cover glass. FIG. 7 shows such a design. It is a packaged CMOS image sensor 700 that includes a cover glass 710, a CIS device 730 that comprises microlens (not shown), substrate, photo detection regions (not shown), and metal connect regions (not shown), dams 740, and solder balls 750. The cover glass 710 is coated on both sides with a top-side AR coating 721 and a bottom-side AR coating 722.

A multi-layer AR coating may be formed onto a bare glass substrate in steps as laid out in FIG. 8. First in step 810, a bare glass substrate is cleaned and dried. Then, in step 820, the bare glass substrate is coated with a first layer of a first index material. For example, the glass substrate is coated with a first layer of a high refractive index material tantalum oxide. Next, in step 830, a second layer of a second index material is added. For example, a second layer of a low refractive index material silicon oxide may be added on top of the first layer. Alternatively, the first layer may be a low index material such as silicon oxide, and the second layer may be a high index material tantalum oxide.

In the next set of steps 840, alternating layers of the first and second index materials are consecutively formed on top of each other, so as to form a multi-layer AR coating. For example, alternating layers of tantalum oxide and silicon oxide are consecutively formed on top of each other. The final layer, which is the outermost layer at the air interface, may be a nineteenth layer of tantalum oxide. Notably, this layers thickness may have a range of 1-2 nm. After the formation of the multi-layer AR coating, this coating is cleaned and dried in step 850, and then inspected in step 860.

Forming each high or low index layer may be accomplished by a coating apparatus that operates with methods such as vapor deposition and sputtering. Generally speaking, vapor deposition is not as precise as sputtering in order to control coating layer thickness. This is partly because in vapor deposition, the distance between a target (i.e., source of metal) and substrate (i.e., bare glass substrate) is often more than 120 cm. Therefore, even after deposition is stopped, ionized atoms in the 120 cm space will continue to deposit onto the substrate for some time, causing the coating layer to be thicker than the desired value. Sputtering does not have this issue, and is the preferred method to form a thin layer of AR coating, e.g., the 1-2 nm, outermost Ta₂O₅ layer at the air interface.

For a coating apparatus (e.g., a sputter coater), a layer thickness monitor is an essential element. This may be a quartz crystal monitor, which operates based on the principle of a quartz crystal microbalance. When individual atoms deposit onto a quartz crystal oscillator, its oscillation frequency may change accordingly. This quartz crystal oscillator may be pre-calibrated, together with an actual coating substrate such as a bare glass substrate, to establish the relationship between oscillation frequency and coating layer thickness. This post-calibrated quartz crystal oscillator is then incorporated as part of the layer thickness monitor. In this manner, a high level of coating accuracy, e.g., 0.001 angstrom per second, may be achieved. A layer thickness of 1-2 nm may be accomplished by using a coating apparatus that utilizes this type of layer thickness monitor. Error range of coating thickness is controlled to be within 5-10%.

The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the exact forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. These modifications can be made to the invention in light of the above detailed description.

The terms used in the following claims should not be interpreted to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are interpreted in accordance with established doctrines of claim interpretation. 

1. An optical element comprising a transparent substrate and an anti-reflective coating, wherein the anti-reflective coating further comprises: at least a transparent, high refractive index layer and a transparent, low refractive index layer, wherein the high refractive index layer is in contact with the low refractive index layer; and wherein the high refractive index layer is situated at an interface between the anti-reflective coating and air.
 2. The optical element of claim 1, wherein the low refractive index layer is a silicon dioxide layer.
 3. The optical element of claim 2, wherein the high refractive index layer is a tantalum oxide layer.
 4. The optical element of claim 2, wherein the high refractive index layer is a silicon nitride layer.
 5. The optical element of claim 2, wherein the anti-reflective coating comprises seventeen, additional alternating layers of transparent high refractive index and low-refractive index materials.
 6. The optical element of claim 5, wherein the substrate is in contact with a first layer of low refractive index material.
 7. The optical element of claim 6, wherein the high refractive index layer has a thickness that has a range that is substantially equal to 1-2 nm.
 8. The optical element of claim 1, wherein the substrate is a flat cover glass.
 9. An image sensor package comprising a CMOS image sensor and an optical element, wherein the optical element further comprises a transparent substrate and an antireflective coating, wherein the anti-reflective coating further comprises: at least a transparent, high refractive index layer and a transparent, low refractive index layer, wherein the high refractive index layer is in contact with the low refractive index layer; and wherein the high refractive index layer is situated at an interface between the antireflective coating and air.
 10. The image sensor package of claim 9, wherein the low refractive index layer is a silicon dioxide layer.
 11. The image sensor package of claim 10, wherein the high refractive index layer is a tantalum oxide layer.
 12. The image sensor package of claim 10, wherein the high refractive index layer is a silicon nitride layer.
 13. The image sensor package of claim 10, wherein the anti-reflective coating comprises seventeen, additional alternating layers of transparent high-refractive index and low-refractive index materials.
 14. The image sensor package of claim 13, wherein the substrate is in contact with a first layer of low refractive index material.
 15. The image sensor package of claim 14, wherein the high refractive index layer has a thickness that has a range that is substantially equal to 1-2 nm.
 16. The image sensor package of claim 9, wherein the substrate is a flat cover glass.
 17. An optical element comprising: a transparent substrate; a first layer of transparent, low refractive index material that is in contact with the substrate; a second layer of transparent, high refractive index material that is in contact with the first layer; additional, alternating layers of transparent low-refractive index and high-refractive index materials; a nineteenth layer of transparent, high-refractive index material that is situated at an interface between the nineteenth layer and air.
 18. The optical element of claim 19, wherein the nineteenth layer has a thickness that has a range that is substantially equal to 1-2 nm.
 19. The optical element of claim 18, wherein the nineteenth layer is a tantalum oxide layer.
 20. The optical element of claim 18, wherein the nineteenth layer is a silicon nitride layer. 